This invention relates generally to a computer controller monitoring and control system for a welder and more particularly for a monitoring and control system for calculating the impedance of a welding load connected to a pulse welder and utilizing this impedance information to determine the internal temperature of the weld and thus determine the quality of the nugget developed during the weld. This is accomplished by generating or absorbing certain counting pulses which are utilized to control the duration of the heat and cool time during the welding cycle.
As is commonly known, a spot weld is created by passing large current pulses, in the case of a pulse welder, through the two pieces of metal to be welded. This current causes heating at the work-to-work interface, the bulk of the available energy being dissipated at this interface due to the fact that the highest electrical resistance point occurs at the interface. Other points in the electrode-to-electrode assembly may dissipate some energy due to resistance between the electrode and work, and certain other factors. Most conductive materials have a resistivity that depends on the material temperature. Thus, as the temperature increases, the resistivity also increases. Therefore, the resistance of a weld nugget is an indicator of its temperature and the impedance history of a spot weld provides an insight into the distribution of energy dissipated during the creation of that weld. Accordingly, by using selected instrumentation that allows for accurate measurement of the weld impedance, a non-destructive determination of weld quality may be made during the formation of the weld.
The resistance of a weld during the course of the weld can be approximated as the solution to an integral equation. This equation is derived here and the solution is obtained. Two assumptions are required to define the problem.
1. Heat loss from the weld is proportional to the time integral of the temperature difference between the weld and the heat sink. PA1 2. Temperature rise is proportional to net input energy. PA1 T.sub.o =initial system temperature PA1 T.sub.i =heat sink temperature PA1 ei=power input to the weld PA1 k=the ratio of thermal conductivity to specific heat PA1 s=reciprocal of specific heat
These assumptions lead to the following integral equation which describes the weld temperature rise. ##EQU1## where T=weld temperature
Material resistivity varies with temperature according to the following expression, EQU .rho.=.rho..sub.o (1+.eta.T)
where .rho..sub.o is the resistivity of the material at zero degrees centigrade and .eta. is a physical constant of the material being tested. Initial resistivity is converted into resistance by EQU R.sub.o =.rho..sub.o (L/A)
where L is the path length and A is the cross sectional area of the conducting path. It follows that, if a constant current source is used for the weld, ##EQU2## A solution to this equation is found with the aid of Laplace transforms to be ##EQU3## This solution is verified by substitution of it into Equation 2. The equation must be examined to determine if it satisfies all physical requirements. Under the condition that I=0, the temperature becomes EQU T=T.sub.i +(T.sub.o -T.sub.i)e.sup.-kt ( 5)
which indicates that the system temperature approaches the heat sink temperature. From Equation 4, an unstable condition appears to arise when s.eta.I.sup.2 R.sub.o is equal to k. However, further analysis shows that the solution is stable at this condition. This stability is shown by L'Hospital's rule. Let EQU M=sI.sup.2 R.sub.o +kT.sub.i EQU N=k-s.eta.I.sup.2 R.sub.o
and Equation 4 becomes ##EQU4## L'Hospital's rule shows that ##EQU5## which indicates that s.eta.I.sup.2 R.sub.o =k corresponds to the condition that added energy exactly matches energy lost to the cooling system so that no temperature change is observed. This condition cannot occur unless .eta. has a non-zero value. Otherwise the temperature must either rise or fall at an exponential rate. The remaining case where EQU k&lt;s.eta.I.sup.2 R.sub.o
corresponds to the induction of heat at a rate so great that the heat sink is unable to establish a maximum temperature. Here the weld temperature will increase without bound until stopped by self-destruction. This case is not of interest in this analysis.
The terms M/N are temperature equivalent terms that depend on both material and weld parameters. When EQU k.ltoreq.s.eta.I.sup.2 R.sub.o
the term represents the end point temperature that the weld will approach and the weld can either cool or heat depending on the end temperature magnitude. Therefore let ##EQU6## and the weld temperature becomes EQU T=T.sub.f +(T.sub.o -T.sub.f)e-(k-s.eta.I.sup.2 R.sub.o)t (6)
It is therefore established that Equation 6 is a solution to the original integral equation that meets all physical requirements, and the resistance of the weld is written EQU R=R.sub.o (I+.eta.T.sub.f +.eta.(T.sub.o -T.sub.f)e-(k-s.eta.I.sup.2 R.sub.o)t) (7)
For small values of (k-s I.sup.2 R.sub.o) this expression can be approximated by EQU R=R.sub.o (I+.eta.T.sub.o +.eta.(T.sub.f -T.sub.o)(k-s.eta.I.sup.2 R.sub.o)
which is abbreviated to EQU R=At+B
where EQU B=R.sub.o (I+.eta.T.sub.o) (8)
and EQU A=R.sub.o .eta.(T.sub.f -T.sub.o)(k-s.eta.I.sup.2 R.sub.o) (9)
Equations 8 and 9 describe completely the transient impedance of a weld.
The weld time is broken into segments of heat and cool times. At the beginning of each time period, the initial temperature is T.sub.o. During a heating impulse, Equation 8 describes the rise in resistance with time. During cool the form of Equation 9 changes to EQU A=R.sub.o .eta.(T.sub.i -T.sub.o)k
which is obtained from Equation 5 or equally well from Equation 9 when I=0.
There are three distinct real time corrections that can be applied to a weld controller to improve weld quality. Each correction can be applied individually or in combination with others depending on the magnitude of the correction needed on the design of the particular controller.
It follows from Equation 6 that during the heat cycle, the weld temperature traverses an exponentially damped transition from the initial nugget temperature toward T.sub.f. These boundaries will not be altered by an adjustment of heat time, but rather the temperature will progress to a different value along the same prescribed path. A derivative approximation is used to estimate the effect of the heat time adjustment. Assume that (k-s.eta.I.sup.2 R.sub.o) is small and the time rate of change of Equation 6 becomes EQU .DELTA.T.sigma.(T.sub.f -T.sub.o)(k-s.eta.I.sup.2 R.sub.o).DELTA.t (10)
and the change in R is merely EQU .DELTA.R=R.sub.o (T.sub.f -T.sub.o)(k-s.eta.I.sup.2 R.sub.o).DELTA.t (11)
The above equations demonstrate two factors. If the heat time in a pulse welder is increased (by a small amount relative to the initial heat period), the end point temperature rise will increase linearly with respect to the heat time and the increase in resistance is also linearly proportional to the increase in heat time. It is to be noted from a later discussion that the beginning of weld phase II is marked by a minimum beginning impulse temperature in excess of the melting temperature of the material being welded.
The object of the control system is to force the resistance or temperature of the weld to follow a prescribed path throughout the weld. It can therefore be safely postulated that corrections applied will follow the theoretical formulation.
Three types of corrections are evident; current or phase adjustment, heat time and cool time adjustment. The curves shown in FIG. 1A demonstrate the effects of altering these parameters on the weld temperature, or resistance, for a particular impulse. Modification of phase or current raises or lowers T.sub.f correspondingly. The effects of this type of parameter modification is shown in FIG. 1A. Because the exponential rise rate is not affected only slightly by a change in T.sub.f, only a small change in peak temperature and similarly in the weld temperature at the end of the cool cycle is obtained. Such a correction is a good vernier adjustment but has limited application as a feed-back control system. If a measurement determines that the weld progress is not within acceptable limits it is desired that a correction be made to the impulse being sampled. Phase correction can be made immediately upon determination of the requirement; however, time is required to find if the correction is necessary. Effects of this delay are shown in FIG. 1B. Here note that the end effect in temperature is reduced in proportion to the delay in beginning of the correction.
Heat time corrections effects are shown in FIG. 1C. Note here that the effect of increasing heat time is relatively small when compared with the effect of shortening the cool time. Therefore to slow progress of a weld, shortening heat time has a significant effect.
In FIG. 1D there is shown the effects of altering the cool time. Note here that the effect is similar in magnitude if the cool time is either lengthened or shortened. From the standpoint of maximum production efficiency, it is desired that the weld time always be shortened rather than lengthened. Therefore, heat should be added to the system by shortening the cool time, and heating should be slowed by shortening the heat time.
All three of the available weld parameters alter the temperature or resistance history of the weld in a manner that would be expected by intuition. Choice of the most appropriate correction depends upon the system on which the loop is being closed. In the system of the present invention, it has been found desirable to make the corrections in the early part of the weld cycle. In resistance spot welding, the weld nugget is formed in two phases, the first phase being utilized to bring the work up to the melt temperature of the work material and the second phase being utilized to maintain that temperature to permit the nugget to grow. Accordingly, as the impulses during the heat portion of the cycle are sensed, the impedance is calculated and compared to the preselected desired curve and corrections are made to insure that the temperature of the weld, as it is being brought up to the preselected temperature, fits that curve. It is during this first phase that the corrections are generally made.
The system to be described in presenting the concepts of the present invention include a standard pulse welder controller presently available on the market. This standard controller typically comprises a system for counting impulses during alternate heat and cool cycles and a circuit for switching the control of the energy from heat to cool in an alternate fashion. The system of the present invention is adapted to be interconnected with the standard pulse welder to sense when a heat portion of the cycle is occurring and when a cool portion of the cycle is occurring. The novel system then determines whether a correction is to be made and whether that correction should be such to apply more or less heat to the work interface. If it is determined that more heat should be applied in a typical situation, the cool cycle will be shortened by generating one or more additional cool impulses within the novel system and feeding these additional cool impulses to the standard welder control to be counter by that standard control. Thus, the standard control is fooled into counting a pulse generated outside of the standard control to shorten the cool cycle. In the case of lengthening the cool cycle, one or more of the cool impulses is precluded from being counted by the standard control. This could occur by shunting a cool impulse or by maintaining a voltage level at the input circuit at the counter at a particular level such that it appears that an impulse has not been generated when in fact one or more impulses have been generated in a circuit prior to the counter.
The novel system includes any general purpose, digital computer which is fed data from the welder heads and the computer is utilized to generate output commands to the standard controller. In the preferred embodiment, an Alpha 16 minicomputer manufactured by Computer Automation, Inc. is used. In one embodiment of the invention, voltage sensing leads are interconnected with the welder electrodes to sense the voltage across the work. Also, additional connections are made to accurately sense the current flowing through the work, these measurements being made by standard methods such as standard shunts connected to a portion of the welder load circuit, etc. It is presently contemplated that the voltage will be measured first and then stored for a short period of time until the current is measured. These signals are fed through an analogue to digital converter and control circuit to the computer, the voltage and current data being fed to the computer and control signals being fed from the computer to the converter to control when the voltage and current are sensed. The computer then calculates the impedance of the weld and produces control signals for use by the standard welder controller.
In a typical system, the computer is ordinarily set up to sense a preselected number of load welds, the welds being controlled to produce a desired nugget characteristic. The computer then calculates the impedance for these welds, the number of welds being in the neighborhood of fourteen for each head being controlled and generates the desired mean curve. The computer then calculates standard deviations for that curve to set up maximum and minimum deviation limits which are acceptable for welds to be performed under the standards set up. The computer then compares future welds with the standards programmed into the computer to determine whether a correction is to be made and whether a correction can be made (the weld is out of limits). The computer also includes a system for updating and weighting additional data being fed during the future welds to permit following of the weld standards to any drift in the welding cycle which may occur as a result of deterioration in the weld electrodes or some other subtle drift in the weld quality.
The system further includes a welder interface circuit for interfacing the computer control signals with the standard welder control. Further, the system includes diagnostic circuits for indicating when a correction is being made and for indicating when a correction cannot be made to mark the work for discard or reworking.
Accordingly, it is one object of the present invention to provide an improved welder control system.
It is another object of the present invention to provide an improved resistance pulse welding control system.
It is another object of the present invention to provide an improved control system for use in conjunction with a standard impulse welder control system.
It is a further object of the present invention to provide an improved system for calculating the impedance characteristics of a weld as it is being accomplished and comparing the calculated impedance characteristics with the preselected characteristics to determine the progress of the weld.
It is still another object of the present invention to provide an improved pulse welder control system which is capable of comparing weld characteristics with a preselected characteristic standard and applying corrections to the welder to force the weld characteristics to conform to the preselected standard characteristics.
It is still a further object of the present invention to provide an improved impulse welder control which is capable of controlling a multiple number of welder heads and calculating the weld impedance as each impulse of weld current is applied to the work.
It is still a further object of the present invention to provide an improved system such as described in the previous object and apply corrective action to the weld after each weld impulse is applied to the weld to insure that the weld conforms to preselected standards.
It is still another object of the present invention to provide an improved welder control system as described which has the capability of applying corrective action to the weld as it is being applied to the work by shortening or lengthening either the cool or heat portion of the weld cycle to insure that the temperature of the weld follows certain preselected standards.
It is another object of the present invention to provide an improved resistance welder control system which is capable of indicating when corrective action is being taken and when the weld is such that corrective action cannot be taken.
It is a further object of the present invention to provide an improved pulse welder control system which is inexpensive to manufacture, easily installed and reliable in operation.
It is a further object of the present invention to provide an improved welder control system for correcting standard weld procedures and which is capable of being interconnected with standard resistance welding control circuits.